This invention relates to the identification and characterization of a new microorganism isolated from a patient suffering from ehrlichiosis. The new organism, designated herein as Ehrlichia chaffeensis, is similar to but distinct from Ehrlichia canis. 
Human ehrlichiosis is a newly recognized disease characterized by fever, headache, malaise, thrombocytopenia, leukopenia, and elevated liver enzymes (Anon., M.M.W.R. 37, 270, 275, 1988; Fishbein, et al., JAMA 257, 3100, 1987; Fishbein, et al., J. Infect. Dis. 160, 803,1989; Eng, et al., JAMA 264, 2251, 1990). Often the patients also have a history of tick exposure. The only Ehrlichia species known to infect humans is Ehrlichia sennetsu, the agent responsible for sennetsu rickettsiosis, a disease that has been reported only in Japan and Malaysia (Ristic, in Microbiology 1986, L. Leive, Ed., American Society for Microbiology, Washington, D.C., 1986, pp. 182-187). Since recognition of a human form of ehrlichiosis in the United States in 1986, laboratory-based surveillance has led to the identification of about 215 persons with variable antibody titer to E. canis in 20 states, predominantly in southeastern and south central areas of the United States (Fishbein, et al., J. Infect. Dis., 160, 803, 1989; Eng, et al., JAMA 264, 2251, 1990). It may be noted, however, that despite such serologic evidence, the causative agent of human ehrlichiosis remained unidentified and the etiology of the disease also remained undetermined.
It is, therefore, an object of the present invention to isolate, identify and characterize the agent associated with human ehrlichiosis, the agent thus isolated having been designated herein as xe2x80x9cEhrlichia chaffeensisxe2x80x9d or xe2x80x9chuman Ehrlichianxe2x80x9d.
It is noted that if the scientific community accepts the change of nomenclature of E. chaffeensis to E. homosapiensis or other designation, then of course it should be recognized accordingly.
It is another object of the present invention to grow the Ehrlichia chaffeensis isolate in a cell culture.
It is also an object of the present invention to provide a recombinant molecule or construct containing E. chaffeensis nucleotide sequence or E. chaffensis-specific fragment thereof.
A further object of the present invention is to prepare antibodies having specificity particularly against E. chaffeensis. 
A still further object of the present invention is to provide cloned genes of E. chaffeensis that encode E. chaffeensis-specific antigens.
An additional object of the present invention is to provide a composition comprising an immunogenic amount of E. chaffeensis antigen, either naturally produced or recombinantly made, to induce antibodies against E. chaffeensis in a host susceptible to infection by E. chaffeensis. 
A further object of the present invention is to provide an immunoassay for detecting human ehrlichiosis employing E. chaffeensis or a fragment derived therefrom as an antigen.
Another object of the present invention is to provide a diagnostic kit comprising a container containing E. chaffeensis-specific antigen or antibody.
Yet another object of the present invention is to provide a method for screening the toxicity of a drug against E. chaffeensis by comparing the growth of E. chaffeensis in the presence and absence of the drug in a cell culture environment.
Also provided herein is an immunoassay for detecting Ehrlichia chaffeensis or an antigen of Ehrlichia chaffeensis, comprising reacting a sample suspected of containing Ehrlichia chaffeensis or an antigen of Ehrlichia chaffeensis, with an antibody having specific binding affinity to an antigen of Ehrlichia chaffeensis, and determining the occurrence of an immunological reaction between said sample and the antibody.
Various other objects and advantages will become evident from the following detailed description of the invention.